patch 9.1.0891: building the completion list array is inefficient

Problem:  building the completion list array is inefficient
Solution: refactor and improve ins_compl_build_pum() func
          (glepnir)

current time complexity is O(n^2). I guess garray is not used here to save memory and avoid efficiency
is caused by heap memory allocation. A simple way is to add an extra pointer as a single linked list
to store the matching compl_T, and traverse this single linked list to generate compl_match_array.
The time complexity is O(n x m). The worst case is m=n, but we can still get a little improvement.
Because the if condition does not need to be run at one time. This should be a good solution for now.
Later we may be able to complete it in O(lgn) time. But this requires more reconstruction. So this is
the first step.

closes: #16125

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    891,
 /**/
     890,
 /**/